1. The Field of the Invention
The present invention relates to a power supply circuit with a series regulator.
2. Related Art
Power supply circuits, which are required by almost all electronic apparatuses, can be categorized into a large number of types, one of which is a series-regulator type of power supply circuit.
FIG. 1 exemplifies the electronic configuration of a series-regulator type of power supply circuit, which has typically been used by in-vehicle electronic equipment, such as ECU (Electronic Control Unit).
The power supply circuit 1 shown in FIG. 1 has a supply circuit 4 (main power supply) to which a voltage VS is supplied from a battery 2 via an ignition (IG) switch 3 and a second supply circuit 5 (auxiliary power supply) to which the voltage VB is supplied directly from the battery 2. Outputs of both supply circuits 4 and 5 are connected to a common output terminal 6 connected to a load circuit 7. The input side of the supply circuit 4 is connected to a second load circuit 8. The supply circuits 4 and 5 include main transistors 9 and 10, respectively. The emitter and a base of each main transistor 9 (10) are connected to its input and output. These two-systemized supply circuits 4 and 5 individually compose series regulators that operate on mutually-different target output voltages.
This series-regulator type of power supply circuit 1 operates as follows. When the ignition switch 3 is in the on-state, the supply circuits 4 and 5 both work, so that the voltage Vo at the output terminal 6 is stabilized to either one, which is higher than the other, of the target output voltage of the supply circuit 4 or that of the supply circuit 5. Meanwhile, when the ignition switch 3 is in the off-state, the supply circuit 5 operates alone, so that the voltage Vo at the output terminal 6 is stabilized to the target output voltage of the supply circuit 4.
In the latter case, the base and collector of the PNP-type transistor 9 are inserted into the circuit in the forward direction. Therefore, though it depends on how the load circuit 8 is configured, it may happen that current flows in the backward direction from the supply circuit 5 to the load circuit 8 via the collector and base of the transistor 9 and the resistor 11.
In order to avoid such a backward direction current, a conceivable countermeasure is to place a diode between the ignition switch 3 and the transistor 9. However, placing the diode there gives rise to a decrease in the input voltage to the supply circuit 4 correspond to the forward voltage Vf of the diode, thus providing a swell in a minimum operating voltage to the battery voltage VB.
The problem of this flow of backward current is not always inherent to the configuration where the two-systemized supply circuits 4 and 5 use a common output terminal. Such a problem may arise even in one-system power supply circuits, as long as there is a possibility that the power supply circuit is subjected to an inverse application of voltage from the load circuit 7.